An essential clue in trouble shooting are log files. Our Communardo Support asks in some cases for the Confluence log.
If you have not changed the destination of the log files in your Confluence, you find your log at your Confluence server:
<Confluence Home Directory>/logs/atlassian-confluence.log
If you don't know your Confluence Home Directory, you can look it up in your Confluence Administration -> System Information. There you can find an entry called "Confluence Home".
In most recent Confluence versions, you can alternatively create a support zip with all irrelevant options unchecked (you only need Confluence Application Logs) and download it directly in the administration.
Please note, that log files can be very large. If you attach the file to your support ticket, it must not be larger than 10 MB.